Under the node of Tools>Field Functions, you can creating your own field function by right-clicking this node and selecting to create a new one. But note that, STAR CCM has its regulation to write a field function, so please consult the User's guide to resolve your problem. Hope that help.

thanks for your reply, but my question was not how to setup a field function in general. I was looking for a "numerical" formulation of the entropy which is calculated in every point. I want to see the Entropy for example over the pressure side of a airfoil,...